1. Field of the Invention
The present invention relates to a tuner circuit for use in devices such as a television set, a video tape recorder, a set-top box and others, and also relates to a digital broadcast receiver.
2. Description of the Background Art
In recent years, terrestrial and CATV broadcasts have been moving from current analog broadcasts to digital broadcasts because the digital broadcasts can increase the number of channels, and can perform image processing of high quality. Therefore, digital broadcast receivers for receiving digital broadcasts are now in increasing demand.
FIG. 9 is a schematic block diagram of a tuner circuit in a conventional digital broadcast receiver.
In a conventional tuner circuit 200 shown in FIG. 9, a received signal received by an antenna 1 undergoes band limiting by input filter 2, and then is supplied to a variable amplifier 3 which may also be referred to as an “RF-AGC amplifier 3”.
The received signal amplified by variable amplifier 3 undergoes band limiting by an interstage filter 4 (which may also be referred to as a “SAW” filter”), and is supplied to a mixer circuit 5. Antenna 1, input filter 2, variable amplifier 3 and interstage filter 4 form a receiver unit 30.
A PLL circuit 7 can change an oscillation frequency of a local oscillator 6 with a tuning voltage, and this local oscillator 6 produces a local oscillation signal interlocked with the tuning voltage. It is assumed that a digital demodulator circuit 22 controls digital PLL circuit 7.
Mixer circuit 5 mixes the local oscillation signal with the received signal to perform frequency conversion, and provides a signal to an IF amplifier 8. IF amplifier 8 amplifies the received signal, and provides it to an interstage filter 9. After band limiting is effected by interstage filter 9, the received signal is amplified by a variable amplifier 10 (which may also be referred to as an “IF-AGC amplifier 10”, and is provided to a demodulator unit 20. Mixer circuit 5, local oscillator 6, PLL circuit 7, IF amplifier 8, interstage filter 9 and IF-AGC amplifier 10 form a frequency converting unit 40. Each of RF-AGC amplifier 3 and IF-AGC amplifier 10 may be collectively referred to as “AGC amplifier”.
Demodulator unit 20 includes an A/D converter circuit 21 (which may be simply depicted as “A/D”) converting the amplified signal amplified by IF-AGC amplifier 10 to a digital signal, a digital demodulator circuit 22 demodulating a digital modulated signal by QAM demodulation, OFDM demodulation or the like, and an error correcting circuit 23 which corrects an error caused by noises or the like. Digital demodulator circuit 22 performs conversion to provide TS (Transport Stream) signals including a picture signal, an audio signal, a data signal and others.
Internal circuits of the digital broadcast receiver converts the TS signals to the picture, audio and data signals, which can be viewed and listened to on a picture display device such as a television set.
An AGC (Auto Gain Control) operation for the received signals (including a disturbing signal) will now be described.
The AGC operation is performed to achieve automatically an optimum performance according to an intensity level of the received signal (including a disturbing signal) and more specifically to adjust a signal level. Generally, the RF-AGC amplifier for the RF signal and the IF-AGC amplifier for the IF signal are used.
This example is provided with an AGC control unit 24, which supplies AGC voltages to AGC amplifiers 3 and 10 in response to an instruction of digital demodulator circuit 22. In RF-AGC amplifier 3, a gain of the AGC amplifier is raised to prevent NF deterioration when the received signal is weak. When the received signal is strong, the gain of the AGC amplifier is lowered to adjust the AGC voltage for preventing deterioration of a distortion performance.
In IF-AGC amplifier 10, the input level of the A/D converter circuit is likewise changed according to the intensity of the received signal so that the digital signal in the optimum state is provided to the digital demodulator circuit to obtain the optimum demodulation performance.
More specifically, RF-AGC and IF-AGC amplifiers 3 and 10 are supplied with the AGC voltages corresponding to the level of the received signal provided to digital demodulator circuit 22, and thereby adjust the signal levels to improve the reception sensitivity and thus the reception performance. Although AGC control unit 24 shown in FIG. 9 is independent of digital demodulator circuit 22, AGC control unit 24 may be arranged within digital demodulator circuit 22, or may be arranged outside demodulator unit 20.
FIG. 10 illustrates filter characters of interstage filter 9.
As shown in FIG. 10, interstage filter 9 is designed to allow passing of a frequency of a digital signal included in a reception band. Therefore, the interstage filter 9 substantially removes the disturbing signal and others having frequencies outside the allowed range or band. Therefore, the degree of intensity of the disturbing signal is not taken into consideration, and digital demodulator circuit 22 of demodulator unit 20 operates to perform the AGC control using AGC control unit 24. Accordingly, when an excessive disturbing signal is input, the waveform of the received signal downstream from the AGC amplifier may be distorted so that the reception performance may deteriorate.
FIG. 11 is a schematic block diagram of another tuner circuit inside the conventional digital broadcast receiver.
Referring to FIG. 11, a tuner circuit 210 differs from tuner circuit 200 in that an AGC control unit 15 (which may be depicted merely as “AGC”) controlling RF-AGC amplifier 3 is employed in addition to AGC control unit 24 controlling IF-AGC amplifier 10. Other structures are substantially the same as those in FIG. 9, and therefore detailed description thereof is not repeated.
As shown in FIG. 11, AGC control unit 15 produces and supplies the AGC voltage to RF-AGC amplifier 3.
Since tuner circuit 210 undergoes the AGC control with the signal not yet passed through interstage filter 9, the amplification level of the RF-AGC amplifier is adjusted also depending on the degree of intensity of the disturbing wave signal, and it is possible to provide the tuner circuit having the reception performance which does not deteriorate even when the disturbing signal becomes strong.
In this case, when an excessive disturbing signal is input, the gain of the AGC amplifier is lowered to suppress distortion of the received signal waveform downstream from the AGC amplifier. However, when the disturbing signal becomes larger than the received signal, the AGC control is executed to lower the gain of the AGC amplifier even when the disturbing signal has the signal level which is not considered as an excessive disturbing level, and this AGC control may cause unnecessary deterioration of the NF.
Japanese Patent Laying-Open No. 06-153100 has disclosed a structure of a tuner circuit which detects neighboring channel disturbing, and restricts the AGC voltage to prevent deterioration of the reception performance when the level of the disturbing signal is high.
However, Japanese Patent Laying-Open No. 06-153100 has merely disclosed the structure of the tuner circuit which restricts the AGC voltage when the disturbing signal is at a level of a predetermined value or more, and has not disclosed a structure which adjusts the AGC voltage according to the disturbing signal.